TROUBLE with PAINT Blistering of Paint Films on Metal Part Osmotic Blistering his months - PDF document

TROUBLEwith PAINTon Metal, Part 1: his monthÕs column will dis-begins a two-unit review of blister-duce them. At this stage, our discus- Fig. 2 - Mechanism of osmotic blistering of coating films Fig. 3 - Sources, sites, and effects of osmotic blistering in coating filmsby Clive H. Hare, Coating System Design Inc. Fig. 1 - Osmotic blistering of coating on steelFigures courtesy of the author Copyright ©1998, Technology Publishing Company TROUBLE with PAINTthe cause of blistering on metal canthe substrate and there sustain theare associated with the productioning under fresh water conditions;electroendosmotic gradients, pro-and thermal gradients, producinghumid environments. Cathodic blis-tering, produced by the generationassociated with electrical gradientsand is often driven by an externallyconsequence of the application ofIn this article, we will considerthought to be the most prevalentcuss the mechanism of osmotic blis-and the sources of osmotic gradientsfrom the corrosion process, retaineding will be characterized, and os-column that intact paint films areable to water, but impermeable todissolved solids. This model is pre-motic blistering. The phenomenon(Fig. 1) depends on the presence ofOften, the active material is an inor-the external face of the paint film(or system) must be in contact withan aqueous environment that is ei-ther free of or lower in dissolvedmaterial than the environment be-Under such conditions, after waterinterface (e.g., metal substrate).There it may come in contact withand leave the film to dissolve thematerial. Under fresh water condi- continued Copyright ©1998, Technology Publishing CompanyFEBRUARY 1998 TROUBLE with PAINTdownstream side of the film wheremuch higher than is the solute con-centration at the external (or up-through the film towards the con-pressure. This transfer of water oc-the membrane attempt to equili-brate. The mechanism of osmoticIn quantitative studies of the phe-Accompanying this growth is a pro-gressive decrease in salt concentra-External water that is relativelyof osmotic gradients. In sea water,as well as potable water found intanks and stand pipes may be partic-ularly troubling. Rain water and high humidities are more general-ized sources of water of low soluteconcentration, which can result inhigh osmotic pressure. But the dura-tion of the ÒimmersionÓ in simpleexterior environments is normallyHowever, persistent humidity andcondensation in such environmentsThe nature of the solute below thebeen related not only to chlorides,sulfates, and other inorganic sol-particular concern to the protectivecoatings engineer. These materialsfar more evidence of critical thresh-there seems with respect to blister-ing. Estimates of permissible salt lev-els for underfilm rusting vary from) to 500 mg Cl). The subject is reviewed inIn the authorÕs opinion, the searchnot occur is inevitably complicatedby the variety of individual modelspossible. Not only is the relationshipcomplicated by film thickness, butinfinitely more so by film character-istics and the mechanism of corro-for example.) Inhibitor-based sys-ble to these contaminations. Tolera-ble levels will depend upon the typecritical pigment volume concentra-tion (CPVC) ratio, pH of the mi- continued croenvironment beneath the coat-ability to water, oxygen, saponificationresistance, and dielectric constantÑwill all have an effect. Thus, itwould appear that permissible saltlevels for underfilm corrosion resis-lar system are known. Unfortunately,For osmotic blistering alone, thesolute seems most important to thesize and morphology of the blisterlevel. Morcillo et aldifference in the solute concentra-membrane need not be large to sup-port the continued growth of theblister. As is noted by van der Meek-amounts of hydrophilic surface cont-aminants may be sufficient to causeBlistering patterns reminiscent offingerprints have betrayed untowardof perspiration onto the steel byworkers before painting. Most typi-rine environments, bridge deicingand industrial effluent (SOand nitrogen oxides that cause fre-silica sand) have also been noted assubstrates from abrasive was ex-bridge structure surfaces is surpris-Significantly, solubles accidentallydates, and borates within primerfilms may cause osmotic blisteringeither between coats or at the metalIn 1991, the Pittsburgh Society foring could be related to the amountof water-extractable material in thederived as a result of reaction orTROUBLE with PAINT Copyright ©1998, Technology Publishing Company TROUBLE with PAINTdegradation of pigment or binder.attributed the phenomenon to a sol-uble residue derived from the hy- continued Fig. 5 - Corrosion process in osmotic blister Fig. 4 - Phase separation, microvoiding, and solvent entrapment as a cause of osmotic blisteringAS SOLUBILITY OF BINDER IN SOLVENT SYSTEM IS PROGRESSIVELY IMPROVED, PHASE SEPARATION AND MICROVOIDING OCCUR LATER AND LATER IN THE FILM Copyright ©1998, Technology Publishing CompanyFEBRUARY 1998 TROUBLE with PAINTwholesale delamination of films soFunke has investigated the mor-deposited from mixtures of solventsshown that the onset of incompati-bility and phase separation in films¥the type and ratio of the solvents¥the application temperature; and ¥the glass transition temperatureThe morphological structure of thefilm depends on when incompatibil-mulated systems, films pass into thearation set in more rapidly, poten-morphologies. In extreme cases, thedevelopment of incompatibility earlycontain large, open pores. Phaseseparation occurring subsequent totinuous surface layer covering a mi-crovoided interior. As phase separa-tion occurs nearer and nearer to theduced microvoiding, noted as phaseRather, it shifts downwards, occur-ring at deeper layers of the filmnearer to the interface, at which lo-Hydrophilic diluents and marginalsolvents are found primarily withinthese microvoided areas close to theUnder conditions favoring osmo-towards these microcellular inclu-sions adjacent to the interface. Theic pressure differentials is muchgreater than is any tendency of thewater accumulation is progressiveand results in a blister pattern re-used vinyl lacquers for hiscoating systems, the formulationthe exception rather than the rule.valid. Their molecular weight in-sion process (chemical cure and oxi-by increasing film thickness, wherestructure more entrenched. The blis-ters are often found to contain waterIf the film is post baked at temper-posure, the offending solvent maybe released, and the osmotic pres- Related effects may occur from hy-drophilic solvent imbibition in ser-spaces of tankers handling methanolblisters, but only after the tankswater. This blistering was not seenmolecule is small and is widely usedfor just this purpose in paint re-movers.) The retention of water-mis-the film after the tanks are emptiedmotic blistering may be quite unex-epoxy formulators in these days ofwith ketones. Ketone solvents formlatent ketimines with amine curingamine until after the coating is ap-the atmosphere reverses the reac-tion, releasing the amine as the ke-tone evaporates. The rate of dissoci-midity and temperature of applica-tion, and other factors such as pig-which reduce the rate of moisturethe reaction in the lower layers oferogeneous cure with the upper sur-faces curing over the uncured or theVery polar solvents (e.g., ketones)associate quite readily with water.completely cured films of this typefore complete dissociation of the ke-the heterogeneously cured film mayand associate with that ketone, pro-often with coatings developed sincepopular. The phenomenon has beentiation of immersion service. Here,stress effects leading to later splittingCorrosion in the local environmentbeneath an osmotically formed blis-diately, especially if the liquid withinthe blister does not contain depassi-sion product, which may itself havenotes, corrosion is, however, a se-quential process unconnected withcorrosion of the metal beneath theTROUBLE with PAINT gen permeability of the coating. Cor-If the blistering phenomenon re-then the metal beneath the blistermonths. This condition can occureven under accelerated high humidi-ty test conditions designed to accel-Once the blister is formed, corro-sion will occur by general cell activi-by oxygen permeability through thefilm. Where oxygen permeability isand water to the interface, osmoticblistering may well occur in the ab-sence of externally derived salts orsult of the corrosion process. Thisof some site of localized deadhesionCorrosion rates would initially below because of the high resistancewater solution. However, the forma-motic gradients under favorable con-Thus, osmotic blistering in deoxy-power industry, does not producecorrosion within the blister. In nu-vapor phase of the taurus (the cool-containment areas) is flooded withthought to maintain bright, uncor-form under the coating in immersedareas. In this case, the blister growthblistering and leave the system inplace without repair. In Japan whereyears or more in spite of such blis-tering. In the U.S., where similarTROUBLE with PAINT with associated cracking and dead-hesive propagation, reduces the ser-will plasticize the film under wetfilm may easily retain the blister de-formation. As the water is desorbedthe film becomes less plastic. Hygro- Fig. 7 - Underfilm condition after osmotic blister formationÑshowing the deposit of corrosionproduct underneath blister domes (the other side of the interface shown in Fig. 6, from which Copyright ©1998, Technology Publishing CompanyFEBRUARY 1998 TROUBLE with PAINTsoluble species is soluble ferrouscompounds. Corrosion products be-Again, the growing blister becomesto ferric oxidation process consumesthe available oxygen as soon as thelarization of the base of the blister siteand the shift in cathodes to the blister1.L.A. van der Meer-Lerk and P.M.2.L. Igetoft, ÒRelation between Sur-Proceedings of2nd World Bridge Conference3.J. West, ÒThe Relationship Be-4.M. Morcillo, S. Feliu, J.C. Galvan,5.B.R. Alblas and A.M. van Lon-6.M. Morcillo, S. Hernandez, J.Steel Induced by Saline Contami-nants at the Metal/Paint Inter-7.W.C. Johnson, ÒDetrimental Ma-terials at the Steel/Paint Inter-8.B.R. Appleman, S.K. Boocock, R.E.9.H. Gross, ÒExamination of Salt10.W. Wettach and the Pittsburgh So-Study of Factors Affecting the Rust-ing of Steel and Blistering of Or-11.T.R. Bullet and J.L. Prosser, ÒCor-12.A.C. Elm, ÒZinc Dust Metal Pro-13.W. Funke, ÒBlistering of PaintProgress in Organic Coatings14.W. Funke, ÒPreparation and15.Private Communication, G. Tin-16.K.B. Tator, ÒCan Failures Still(for a Given Environment) Is Se-, ed. H. Leidheiser (Hous-17.Private Communication, S.J.